Much the busiest of the Uxbridge area U-routes is the U4. The number derives from route 204 which ran from Hayes to Uxbridge since the war; Bourne Avenue was served earlier, being the terminus of the 140 before that was extended to Heathrow. This section went through a succession of route numbers before becoming part of the U4, but recently the route was diverted to serve Mildred Avenue and Pinkwell Park rather than running the full length of Bourne Avenue.

The U4 can explain its frequency — every 8 minutes during the day on Mondays to Saturdays — to the fact that it almost uniquely serves quite a large area off Harlington Road and Judge Heath Lane. Relatively recent routes A10 and U7 have diminished that to a certain degree, but they cannot compete on frequency.

12 DML class Marshall Capital bodied Dennis Dart SLF buses were purchased for the U4, followed closely by a batch of 10 for route U1. These were amongst the first buses (in London) to sport a new interior colour scheme devised by FirstGroup, apparently to appeal to its female passengers, being in unusual blue/purple tones. This has prompted mixed reactions, with some loving it and others loathing it! To go with it, FirstGroup introduced a new livery in similar colours, with a swish theme below the windows. A London version has also been devised, but nevertheless these vehicles retain the plain red with yellow stripe livery introduced by Centrewest soon after privatisation. Either way, the inevitable red does not coordinate very well with the cool shades inside!

Although devised as a minibus route, and subsequently gaining the larger low floor Dennis Darts mentioned above, the U4 has now been converted to double deck operation to help with capacity issues. Here brand new TN 1329 (LK03 UFE) (now TN 33329) lays over at Uxbridge Station in the summer of 2003.